1. Field of the Invention
This invention relates to aluminum based metal alloys and more particularly to a ductile brazing foil useful for brazing metal articles such as those of aluminum and aluminum alloys and a method for making them.
2. Description of the Prior Art
Brazing is a process of joining metal parts, often of dissimilar composition, to each other. Typically, a filler metal that has a melting point lower than that of the metal parts to be joined together is interposed between the metal parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the filler metal. The molten filler metal reacts with the parts to be joined so that when cooled a strong joint is formed.
Filler metals used are commonly in powder, wire, or foil form depending on the alloy composition and the specific application. The filler metals are often used in conjunction with fluxes and the process may be carried out in inert atmosphere or vacuum, again depending on the filler metal composition and the specific application. The foil form of filler metal provides the advantage of preplacing a controlled amount of filler metal in the joint area, thus permitting brazing of complex shapes with minimum rejection.
Filler metals for brazing of aluminum and aluminum alloys are aluminum-silicon alloys containing 7 to 13 wt % silicon and are designated AWS BA1Si. Liquidus, solidus and brazing temperatures of these alloys decrease with increasing silicon content up to about 12.5 wt %. It has been proposed that these alloys contain up to 3 wt % Mg for fluxless furnace brazing. However, alloys containing high silicon and magnesium contents are extremely difficult to make in foil form because they are brittle and do not respond well to the multiple rolling and annealing cycles necessary to produce foil from ingots or slabs. Filler metal foils are currently limited to relatively low silicon contents of less than 10 wt % with magnesium contents of up to 1 wt %, or to very low magnesium content (0.5 wt %) with more desirable silicon contents. In any case, because of the inherent limitations in the casting and rolling processes, silicon containing particle, hereinafter called silicon particle, size is limited to a minimum of approximately 1 .mu.m and is more typically of the order of 5 .mu.m. Brazing foils with uniformly dispersed silicon particles of less than 1 .mu.m are more desirable than those with large particles. In the commercially available brazing foils there tends to be a relatively heavy oxide film present on the surfaces which is an impediment to the brazing process.
An alternative to filler metal foil is known as brazing sheet. Brazing sheet consists of a brazeable aluminum alloy roll bonded or clad on one or both sides with a brazing alloy of the desired composition.
The limitations in the use of brazing sheet include the increased costs involved in the manufacturing of it. Additionally, there is a minimum thickness of approximately 0.025" for such sheets. The relatively thick sheets, in turn, result in silicon diffusion into the core, which limits the amount of filler metal available for joint formation.
There remains a need in the art for an aluminum brazing alloy with a high silicon content and high magnesium content, the silicon particles being finely dispersed, for low brazing temperatures in fluxless brazing processes. It would be particularly desirable if such aluminum brazing alloy were available in foil form with a minimum oxide film thickness.
U.S. Pat. No. 4,142,571 teaches a method for making continuous polycrystalline or amorphous strips of considerable width (greater than 7 mm) directly from the melt by forcing molten metal onto the surface of a moving chill body under pressure through a slotted nozzle located in close proximity to the surface of the chill body. Because of the density, fluidity, and strong tendency for oxidation of aluminum alloys, it has been found that it is very difficult to maintain melt flow uniformity for long times and therefore produce large quantities of aluminum alloy ribbon suitable for brazing applications with the method as disclosed by Narasimhan in U.S. Pat. No. 4,142,571. There is, therefore, a need for an improved process that would make the production of aluminum brazing foils in commercial quantities possible.